There are a variety of different memory devices available for use in electronic systems. The type of memory device chosen for a specific application depends largely upon what features of the memory are best suited to perform the particular function. For instance, dynamic random access memories (DRAMs) and static random access memories (SRAMs) are used to temporarily store program information and data “actively” being used by a microprocessor or other control device.
Random access memories tend to provide greater storage capability and programming options and cycles than read only memories, but they must be continually powered in order to retain their content. Most dynamic random access memories store data in the form of charged and discharged capacitors contained in an array of memory cells. Such memory cells, however, are volatile in that the stored charges will dissipate after a relatively short period of time because of the natural tendency of an electrical charge to distribute itself into a lower energy state. For this reason, most dynamic random access memories must be periodically refreshed, that is, the stored value must be rewritten to the cells, for example, every 100 milliseconds in order to retain the stored data in the memory cells. Even SRAMs, which do not require refreshing, will only retain stored data as long as power is supplied to the memory device. When the power supply to the memory device is turned off, the data is lost.
Efforts have been underway to create a commercially viable memory device that is programmable, randomly accessed, and nonvolatile. To this end, various implementations of such nonvolatile random access memory devices are presently being developed which store data in a plurality of memory cells by structurally, chemically, or magnetically changing the resistance across the memory cells in response to predetermined voltages respectively applied to the memory cells. Examples of such variable resistance memory devices include those based on polymers, perovskites, doped amorphous silicon, magnetic devices, and chalcogenide glass.
In many variable resistance memory cells, a first value may be written thereto by applying a voltage having a predetermined level to the memory cell, which changes the electrical resistance through the memory cell relative to the condition of the memory cell prior to the application of the voltage. A second value, or the default value, may be written to or restored in the memory cell by applying a second reverse polarity voltage to the memory cell, to thereby change the resistance through the memory cell back to the original level. The second voltage may or may not have the same magnitude as the first voltage. Each resistance state is stable, so that the memory cells are capable of retaining their stored values without being frequently refreshed.
Memory cell structures employing chalcogenide materials as the resistance switching backbone may also be suitably conditioned to exhibit a differential negative resistance (DNR) property. DNR chalcogenide devices have a larger peak-to-valley current-voltage ratio than chalcogenide devices used as memory cells as well as other properties which make them exhibit a differential negative resistance. As a result, DNR chalcogenide devices have greater stability, and hence less power consumption and faster switching speed than conventional devices. The properties and methods of making DNR chalcogenide devices are described in greater detail in U.S. patent application Ser. No. 10/410,567, filed Apr. 10, 2003 to Campbell (Pub. No. US 2004/0202016) and U.S. patent application Ser. No. 10/193,529, filed Jul. 10, 2002, to Campbell (Pub. No. U.S. 2004/0007749), the entirety of which are incorporated herein by reference.
RAM cell densities have increased dramatically with each generation of new designs and have served as one of the principal technology drivers for ultra large scale integration in integrated circuit manufacturing. Typically, SRAM devices consist of at least four transistors. For example, FIG. 1a illustrates a basic six transistor cell with NMOS load devices and FIG. 1b illustrates a four transistor cell with resistor load pull-up devices. With four or six transistor cells, SRAM cell density is limited.
One attempt to reduce the circuit components and thus the size of an SRAM is shown in FIG. 2. A pair of Esaki diodes or resonant tunneling diodes are connected in series; however, their peak-to-valley current-voltage ratio is typically less than 9, thus sacrificing circuit stability for a smaller chip size. Thus, an SRAM circuit having improved stability, non-volatility and decreased power consumption is desirable.